Board Level Reliability Evaluation of Low Silver (ag) Content Lead-free Solder Joints at Low Strain Rates

To improve the durability of lead free solder joints under high strain rates, such as drop and shock loading, some area array manufacturers have converted to low silver (Ag) content tin silver copper (SAC) solder spheres instead of the commonly accepted SAC305 solder. While the lower silver content SAC solder joints may address high strain rate shock loads, the durability of these joints under low strain rates has to be taken into consideration. The objective of this study is to assess the reliability under loading conditions with strain rates several orders of magnitude below that observed in drop testing. To this end, thermo-mechanical loading induced by temperature cycling (-55°C to 125°C) and mechanical loading induced by cyclic board level mechanical torsion were considered. Test vehicles were assembled using SAC305 solder paste with eight peripheral ball grid array packages (BGA) per printed wiring board with SAC105, SAC125Ni and SAC305 solder spheres. Four test specimens per type were assembled. Two of test specimens of each solder combination were then subjected to temperature cycling, while the remaining two were subjected to mechanical torsion. In-situ electrical resistance monitoring was performed on the low electrical resistance paths formed by the individual packages and the printed wiring board. Statistical analysis was conducted on cycles to failure data collected from the defined tests. A drop in fatigue durability with decrease in silver content in the solder spheres was observed for both thermo-mechanical and mechanical loading.